BudsGunShop.com - Will all Glock 19 mags work and Gen 4

William, yes Sir Gen 3 or better will work without issue. Cheers and stay safe! Some great Gen details here : gunpartscorp.com/blog/are-all-glock-magazines-all-interchangeable-100

Yes all (except G26) double stack, 9mm Glock mags will work with this pistol, the only time you'll run into issues is if you need to swap the magazine release for a lefthanded shooter.
As far as differences.
"WHAT WAS UPGRADED IN GEN4?
Backstraps,
Grip Texture, (both the texture and back straps are to help people with smaller hands that felt the grips were to big for their hands)
Magazine Release Button,(larger easier to press on gen4)
Magazine (Gen 4 has amby mag release capability. The Gen3 only for rightys)
Slide (new finish)
Recoil Spring (Gen 4 is the first of the Glock 19 Generations to use a larger, dual-recoil spring guide rod.)
More detailed comparison here
wingtactical.com/glock-19-gen-3-vs-gen-4-differences/